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Post by sandsock on Dec 24, 2011 9:55:41 GMT -8
Pardon me if this was explained in the past but why did the 6.8 go from the original 1 in 10 to the newer 1 in 11? Usually faster rifling is better for heavier projectiles..was the 1 in 10 too fast?
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Post by The Wolverine on Dec 24, 2011 10:51:40 GMT -8
I do not know much about the 6.8 history, but my guess is it did not need to be that fast, and they wanted to push it harder, so they slowed the twist rate, so it could be pushed harder without pressure problems.
The faster the twist rate, the harder it is to push the bullet out of the barrel, so it will build more pressure
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Post by Deleted on Dec 24, 2011 12:34:06 GMT -8
The only reason for the fastest rate is for the subsonic loads. Corbon and SSA has them. I have reloaded a few. They are marginal compared to other subsonic offerings in the market in 308 and 338 calibers but they do the work though.
Subsonic: 130gr Speer SP .... 9.5:1 .... 1207fps. Alliant RL 7.
A good supersonic... I use a 1 in 11.25 SPCII 5R with a 1.0 freebore. A 16" light profile hunting upper pushes a 110gr nosler accubond at 2705fps. I love this light system for hunting in the thick woods.
Merry Christmas.
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Post by floatpilot on Dec 24, 2011 17:10:01 GMT -8
They went to the 1 in 11 and even 1 in 12 twist to reduce chamber pressure with hot loads. The same reason they went to the SPECII chamber design with more free-bore. For some reason when the cartridge first went public there was a lot of thought that the 130 grain bullets would be the standard.
I have stabilized 150 grain flat based bullets with the 1 in 11 twist (Black Hole Barrel) and even most 130 grain boat-tails.
As you know it is bullet length and not weight that matters. For example the 120 grain Barnes Banded Solids. (Great bone perpetrators) do not want to stabilize in my 24 inch BHW barrel. They will just barley stabilize in my old Stag 1 in 10 twist barrel. Not because of the weight but because they are VERY long....
On the other hand, the Hornady SST 120 grain bullets are lead core and thus shorter for their weight. Those work great from my 1 in 11 twist BHW barrel . (2,700 fps) Although they tend not to hold together in heavy animals.
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Post by sandsock on Dec 24, 2011 19:43:30 GMT -8
Thanks everyone and I hope you and yours are having a Merry Christmas.
Besides the 120gr Barnes and the subsonic loads are there other rounds not suitable for 1 in 11? Is there an ideal round?
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Post by Brandon Sneed on Dec 24, 2011 19:45:02 GMT -8
The 6.8 was used un the 1/10 twist because of the bar stock and twist rates for standard 270win. The 10 will stabilize all bullets over 110gr.
That being said, the 6.8 is an intermediate. Not meant to be retrofit to 270win projectiles. This means that anything from 85 to 120 gr for game animals and varmints is the range of effective rounds.
The 11 and 12 twists work wonders for the rifles in 14 to 22" lengths in ARs. Shorters and for suppressed work the 10 and 11 twists seem to be good stuff. I'm working on a 9.5 twist barrel for grins to work 150 like float pilot is in his long barrel but for short barrel suppressed work.
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Post by Deleted on Dec 24, 2011 20:24:43 GMT -8
The 10 twist will not stabilize the solid 150grainers. 1:9.5 is more likely or even 1:9 if suppressed in short barrels is in the menu.
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Post by floatpilot on Dec 25, 2011 1:11:34 GMT -8
As for picking a bullet weight the 85, 90 95 110s and 120s SSTs and the 130 grain flat based Sierra all seem to do well from my rifles. For some reason or another the 100 grain bullets have done well for me in the accuracy dept. But they do not go as fast as you would think they should for their weight. Part of the may be that I have not been able to find any of the new AA-2200 powder up here. But I did hear of one guy who managed o get 3 pounds on a special order.
As an Alaskan my personal desire is to have 2,000 fps at impact and / or 1,000 ft pounds of remaining paper energy.
One big reason I went with a 24 inch Black Hole barrel was to give the 6.5 Grendel guys a run for their money.
So from my 24 inch BHW barreled AR I can push.... 90 grain bullets to 3,120 fps 95 grain Barnes TTSX to 3,090 fps 100s to 2,910 fps 110s to 2,820 fps 120 SSTs to 2,700 fps 130 grain bullets to 2,600 fps.
My personal max hunting range for the above using my long barrel.
90gr HP with BC of .195 = 250 yard max at 2,000 fps impact. 95gr TTSX w/ BC of .296 = 370 yards at 2,000 fps impact 100gr Acubnd. BC .323 = 330 yards at 2,000 fps impact 110gr Acubnd BC .370 = 350 yards both 2,000 fps + and 1,000ftlb + 110 grain Barnes TSX, Same as above 120 SST BC .400 = 340 yards both 2,000fps and over 1,000 ft lbs 130gr Sierra SP, BC .436 = 295 yards over 2,000 fps and 1,100 ft lbs 130gr SST BC . 460= 340 yards over 2,000 fps and 1162 ft lbs
With my 16 inch barrel I loose 200-250 fps compared to the 24 inch BHW barrel.
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Post by Deleted on Dec 25, 2011 8:56:36 GMT -8
LOL! I love it! I might need to order a 24incher in this caliber too!! I finally got rid of my last grendel barrel due to the performance problem it had. I kept the .264LBC due its accuracy. Now this is another "Grendelnator"..... LOL!!!...
6.5BR AR (made from lapua 6mmBR brass) - 130gr Berger VLD 20"Krieger1:8 MV: 2675 gr:130 G1:0.552 Zero: 200 Break speed set at 1300fps past 1100 yards (tested)
Range (Yds) 0 100 200 300 400 500 600 700 800 900 1000 1100 Speed (fps) 2675 2538 2406 2278 2154 2035 1921 1811 1704 1602 1505 1411 Energy (ft.lb) 2065 1859 1670 1497 1340 1196 1065 946 839 741 653 574 Drop (in) 0.00 2.51 10.43 24.36 44.99 73.13 109.68 155.68 212.35 281.08 363.48 461.47 Path (in) -1.50 1.95 0.00 -7.96 -22.63 -44.80 -75.39 -115.43 -166.13 -228.89 -305.33 -397.36 Elevn(MOA) -1.86 0.00 2.53 5.39 8.53 11.97 15.70 19.78 24.22 29.08 34.40 Windage(MOA) 0.25 0.51 0.79 1.08 1.39 1.71 2.05 2.41 2.79 3.20 3.63 Time(Sec) 0.00 0.12 0.24 0.36 0.50 0.64 0.80 0.96 1.13 1.31 1.50 1.71
20". Hot momma but no signs of getting into the red. ...as good as it gets...
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Post by floatpilot on Dec 26, 2011 16:42:14 GMT -8
That is a pretty warm load... How,,,are you able to get the long 130 grain VLDs to seat deep enough that they work in AR magazines without eating up all sorts of powder room???
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Post by Deleted on Dec 26, 2011 17:33:01 GMT -8
Actually there is no much of the bullet going into the case and still not max for powder or pressure. I use regular 6.8pri mags. 6mmBR norma Lapua brass. H4895 . CCI 450 primers. Typical BR accuracy which is phenomenal. I thought about the possibility to neck down the 30RAR but it was too much of a hassle. This one is much easier all the way, including standard 308w bolt face. Now I am working on a 300 Savage. Need to find someone that can ream the AR-15 barrel for me.
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Post by wvhunter1s1k on Mar 19, 2012 19:23:14 GMT -8
A little more hx from what Brandon said. The 1:10 twist was from .270Win bar stock. During the original developing/testing that was all they had to work with.
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